Hardness and case depth measurements are the most important parameters for the quality monitoring of case-hardened steel products and the heat treating process. The current industrial standard technique for these measurements is micro-indentation, which is destructive and time consuming, and therefore not suitable for the need of industrial on-line volume inspection. There have been continuous efforts to search for new methods for evaluating hardness and case depth in a non-contact and non-destructive fashion. In recent years, photothermal techniques have shown strong potential for non-contact and remote hardness and case depth evaluation. A number of photothermal applications to hardness measurements in metals have been reported in the literature. Various independent research groups have reported a well-established anticorrelation between thermal diffusivity/thermal conductivity and microhardness. Jaarinen and Luukkala [see Jaarinen J, Luukkala M., J Phys (Paris) 1983; 44: C6-503] made the first attempt to study the properties of surface hardness of steel in terms of an inverse process and developed a numerical technique based on the solution of the thermal-wave equation using a two-dimensional finite difference grid. Lan et al. [see Lan T T N, Walther H G, Goch G, Schmitz B., J App Phys 1995; 78:4108-4111.] and Mandelis et al. [see Munidasa M, Funak F, and Mandelis A. J App Phys 1998; 83:3495-3498., and Ma T. C., Munidasa M., and Mandelis A., J App Phys 1992; 71, 6029-6035.], showed the capability of photoacoustic (PA) and photothermal radiometric (PTR) detection as depth profilometric techniques for case hardened steels using inverse-problem reconstruction algorithms. Both groups demonstrated anti-correlation between the case depth dependent microhardness and thermal conductivity/diffusivity of the material. Further photothermal radiometric (PTR) studies of hardness case depth profiling were carried out by Walther et al. [see Walther H G, Fournier D, Krapez J C, Luukkala M, Schmitz B, Sibilia C, Stamm H, Thoen J., Anal Sci 2001; 17:s165-168.], Fournier et al. [see Fournier D, Roger J P, Bellouati A, Boue C, Stamm H, Lakestani F. Anal Sci 2001; 17:s158-160.] and Nicolaides et al. [see Nicolaides L, Mandelis A, Beingessner., J App Phys 2001; 89:7879-7884., and Nicolaides L, Mandelis A. J App Phys 2001; 90:1255-1265]. The last group also investigated the microstructure change and the physical mechanisms of the thermal diffusivity depth-profile generation for carburized and quenched AISI-8620 steels. They showed that the variation of thermal diffusivity with depth is dominated by the carbon concentration profile, while the absolute thermal diffusivity values are dominated by microstructural changes occurring during quenching. All those investigations have focused on samples heat treated in the presence of carbon or nitrogen ambient, to form a concentration gradient which subsequently defines the hardness case depth profile after quenching. Recently, the PTR technique was also used in the characterization of non-diffusion controlled steel case depth: The hardness penetration depth of grind-hardened SAE 4140 steel using the calibration curve of case depth versus phase sum [see Prekel H, Ament Ch, Goch G., Rev Sci Inst 2003; 74:670-672.] and the effect of cooling rate on hardness and thermal diffusivity by means of water end-quenched heat treatment in a metallurgical Jominy bar made of AISI 1018 steel [see Liu Y, Baddour N, Mandelis A, Wang C H., J App Phys 2004; 96:1929-1933], were evaluated. As is well known, the PTR signal is sensitive to both thermophysical properties and sample geometry. To simplify geometry effects, all reports to-date concern laboratory based investigations, in which all samples were well defined, prepared and machined flat surfaces with a good finish. Recently, the evaluation of machined cylindrical samples was reported in order to demonstrate the feasibility of the PTR technique with non-flat geometries [Wang C H, Mandelis A, Liu Y., J App Phys 2004; 96:3756-3762. Wang C H, and Mandelis A, Liu Y. J App Phys 2005; 97:014911]. In those studies it was shown that thermal-wave interference occurs in layered curved samples and the details of the interferometric pattern are affected by the degree of curvature.
The present invention is concerned with the capabilities of the photo-thermal radiometric method (PTR) in measuring the effective case depth in case-hardened industrial steels in a non-contact and non-destructive manner. The method and some possible embodiments of the apparatus are presented below.